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In social anthropology, polyandry refers to a form of polygamousmarriage (which simply means "multiple spouses."). Polyandry is the specific form of polygamy in which a woman is married to more than one husband simultaneously. On the other hand, polygyny is when a man has multiple wives, which is by far the more common form of polygamy.

In the field of behavioural ecology polyandry is a type of breeding adaptation in which one female mates with many males. Another similar breeding system to this is polygyny in which one male mates with many females (this is a very common system found in, eg, lions, deer, primates and many systems where there is an alpha male).

A common example of this can be found in the Field Cricket Gryllus bimaculatus of the invertebrate order Orthoptera (containing crickets, grasshoppers and groundhoppers). The unusual thing about polyandry in nature in general is that mating is costly: in other words, why mate with more than one male when you could be better spending your time foraging? Females in this species will mate with any male close to them, including siblings. Possible explanations for polyandry evolving in this species include:
1. It is easier to ensure reproductive success (i.e. it is more likely that the female will have offspring)
2. Females may be encouraging sperm competition between males post-copulation
3. Females may receive food offerings from prospective mates inciting copulation
4. Because males can't be sure if they are or aren't their offspring and won't risk destroying their own DNA, mating with multiple males increases the survival of the female's offspring.
Polyandry also occurs in some primates (eg, marmosets ), other mammal groups (eg, the marsupial "mouse" species Antechinus), some bird species (in around 1% of all bird species, eg, jacanas), insects (such as honeybees), and fish (such as pipefish). In effect polyandry will reduce the effective population size of a given population (closed).

Polyandry in primates and other mammals is usually correlated with reduced or reverse sexual dimorphism. When males of a species are much larger than females, polygyny is usually practiced. As size difference decreases, or the females are larger than males, a species is more likely to practice monogamy or polyandry. The non-human great apes (gorillas, orangutans, and chimpanzees) are highly dimorphic and practice polygyny. Male and female gibbons (lesser apes) are similar in size and form monogamous pairs. Human males and females are less dimorphic in body size than other polygynous great apes, and are often monogamous.

Polyandry is a controversial subject among anthropologists. For instance, Pennsylvania anthropologist Stephen Beckerman points out that at least 20 tribal societies accept that a child could, and ideally should, have more than one father, referring to it as "partible paternity". On the other hand, in Tibet, which is the most well-documented cultural domain within which polyandry is practiced, the testimony of certain polyandrists themselves is that the marriage form is difficult to sustain. However, certain monogamists say the same thing about monogamous marriage[How to reference and link to summary or text].

With particular regard to the supposed failure rate of polyandry (and polygamy in general), it is important to note that there are high rates of infidelity and divorce in "monogamous" societies, so that it is possible to argue that polyandry is not somehow uniquely unworkable. In Tibet polyandry has been outlawed, which means that it is difficult to measure the incidence of polyandry in what may have been the world's most "polyandrous" society.

In other parts of the world, most traditional societies have been drastically altered or destroyed, so the incidence of polyandry in the past may not be accurately known. In India, among Tibetan refugee groups who fled the Chinese invasion of their country, polyandry is seldom encountered.

Some forms of polyandry appear to be associated with a) the perceived need to retain aristocratic titles or agriculturallands within kin groups, and/or b) with frequent male absence, for long periods, from the household. As to the former variety, consider that in Tibet where the practice is particularly popular among the wealthy Sakya priestly nobility as well as poor small farmers who could ill afford to divide their small holdings. As to the latter variety, as some males return to the household, others leave for a long time, so that there is usually one husband present.

The term has gained some currency in sociobiology, where it refers, analogously, to a mating system in which one female forms more or less permanent bonds to more than one male. It can take two different forms. In one, typified by the Northern Jacana and some other ground-living birds, the female takes on much the same role as the male in a polygynous species, holding a large territory within which several males build nests. Subsequently, the female lays eggs in all the nests, and plays little part in parental care. In the other form, typified by the Galápagos Hawk, a group of two or more males (which may or may not be related) and one female collectively care for a single nest. The latter situation more closely resembles typical human fraternal polyandry.

These two forms reflect different resource situations: polyandry with shared parental care is more likely in very difficult environments, where the efforts of more than two parents are needed to give a reasonable chance of rearing young successfully.

Honeybees are said to be polyandrous because a queen typically mates with multiple males, even though mating is the only interaction that they have (the males die off, while the queen uses stored sperm for eggs she fertilizes).

Some New World monkeys, for example Callimicos, have been observed living in polyandrous groups. Although groups may contain more than one female, the dominant female suppresses ovulation in subordinates, causing her to be the only one capable of reproduction. A Callimico female regularly births more than one offspring, and her eggs are separately fertilized by more than one male. Paternal investment is high in Callimicos, and males often carry infants on their backs, even when they are not the father. It has been suggested that multiple male mates were related, and therefore cooperation in caring for each other's young is adaptive; however, researchers tagged and tracked Callimicos over time, and noticed that unrelated males migrated to new groups to cooperate with non relatives as well as with relatives to care for young. It has also been suggested that females select cooperative males, and that the multiple offspring of Callimicos require paternal care for survival.

Current research suggests that Polyandry is the dominant social structure in a subfamily of New World monkeys called Callitrichids, which include Tamarins, Marmosets, and other monkeys.

According to modern feminist theory polyandry is considered by society at large as a relatively taboo subject due to frequent social double standards (sexism) which see female sexuality as inherently inferior to, or less normal than, male sexuality. According to this viewpoint, much of Western thought and literature has focused on finding cause for this behavior that ignores the primary function of female desire. These critics point out that this is articulated most clearly in Victorian morality which inspired Freud's Psychoanalysis which dismisses female desire as a mental health problem. While historical record of formal polyandry is not common, this is also considered to be a circumstance of marriage which is often acknowledged to be patriarchal and thus more suited to polygyny. Judith Butler and Hélène Cixous are two feministcultural theorists whose works have examined female sexuality in regards to the subject of polyandry. They express the idea that polyandry may exist in different forms than what we recognise through simple comparison to polygyny.

Islam bans polyandry completely and the verse from the Quran that is used for a proof in this matter is Surah Nisa’ Chapter 4 verses 22 to 24,which gives the list of women with whom you can not marry and it is further mentioned in Surah Nisa’ Chapter 4 verse 24 "Also (prohibited are) women already married". A woman may not have more than one husband. However, polygyny is allowed, where men can marry up to four wives. Nikah Ijtimah, a pre-Islamic tradition of polyandry, was forbidden by Islam.

Both Judaism and Christianity prohibit polyandry.

There is at least one reference to polyandry in the ancient Hindu epic, Mahabharata. Draupadi marries the five Pandava brothers. This ancient text remains largely neutral to concept of polyandry, accepting this as her way of life.